Microstructural changes of two ${\mathrm{GeO}}_2{-\mathrm{S}\mathrm{i}\mathrm{O}}_2$ glass compositions (with 5 and 13 mol % ${\mathrm{GeO}}_2$) irradiated with 5 and 6.4 eV light have been investigated by Raman spectroscopy. The low-frequency “Boson” bands at $50{\mathrm{cm}}^{\mathrm{-}1}$ of both samples shift upward upon irradiation, but their intensities have opposite changes. It indicates that thermal damage of the surface by energetic UV photons is correlated with the Ge content of the glass. The intensities of both defect lines $D_1$ and $D_2$ increase with respect to ${\omega }_1$ and ${\omega }_3,$ and the ${\omega }_1$ and ${\omega }_3$ bands shift to higher frequencies which means a reduction of the Si-O-Si bond angle upon irradiation. This may be due to the change in ring statistics in favor of smaller rings, that is, sixfold rings transform to threefold and fourfold rings upon UV irradiation. The opposite changes in intensity of ${\omega }_1$ and ${\omega }_3$ bands result from the variation of the network structure. UV photoinduced bond breaking allows structural relaxation of the nonequilibrium glass network that leads to photoinduced Raman changes and the photoinduced index changes in photosensitive glasses.

• Received 2 December 1996

DOI:https://doi.org/10.1103/PhysRevB.56.3066